1. Field of the Invention
The present invention relates to a method for producing ethylene and propylene from a hydrocarbon feedstock by catalytic conversion. More particularly, the present invention is concerned with a method for producing ethylene and propylene from a hydrocarbon feedstock by catalytic conversion, which comprises contacting, in a reactor, a hydrocarbon feedstock comprising 20% by weight or more, based on the weight of the hydrocarbon feedstock, of at least one C.sub.4 -C.sub.12 olefin with a zeolite-containing catalyst to thereby effect a catalytic conversion reaction of the at least one C.sub.4 -C.sub.12 olefin, thereby obtaining a reaction mixture containing ethylene and propylene. In the method of the present invention, not only can ethylene and propylene be produced in high yields, but also the resistance of the zeolite-containing catalyst to deactivation is high, so that the production of ethylene and propylene can be stably conducted for a prolonged period of time. In addition, by the method of the present invention, in the catalytic conversion of a hydrocarbon feedstock, it becomes possible to suppress the by-production of hydrogen, methane, ethane and aromatic hydrocarbons, and improve the selectivity for ethylene and propylene.
Further, the method of the present invention is advantageous in that there is no need to use a reactor system having a complicated system adapted for frequently regenerating the catalyst, and the desired ethylene and propylene can be produced using a simple reactor system, such as a fixed-bed, adiabatic reactor.
2. Prior Art
Various methods for the catalytic conversion of a hydrocarbon feedstock comprised of olefins, in which a zeolite-containing catalyst is employed, are conventionally known. In addition, a number of reports have been made on methods in which a zeolite-containing catalyst is used for producing ethylene and propylene from a hydrocarbon feedstock comprised of olefins by catalytic conversion.
However, for the below-mentioned reasons, it was difficult to stably produce ethylene and propylene efficiently from a hydrocarbon feedstock comprised of olefins by catalytic conversion for a prolonged period of time, using a zeolite-containing catalyst.
Both ethylene and propylene are intermediates of the reaction for converting olefins to aromatic hydrocarbons in the presence of a zeolite-containing catalyst, and these ethylene and propylene are converted to ultimate products, namely aromatic hydrocarbons, by successive conversion reaction.
Therefore, when it is intended to produce ethylene and propylene in high yields from a hydrocarbon feedstock comprised of olefins by catalytic conversion using a zeolite-containing catalyst, the activity of the catalyst and the reaction conditions must be strictly controlled. Illustratively stated, when the catalytic activity is too high and/or the time for contacting the hydrocarbon feedstock with the catalyst is too long, the produced ethylene and propylene are likely to be converted to ultimate aromatic hydrocarbons by successive reactions. On the other hand, when the catalytic activity is too low and/or the time for contacting the hydrocarbon feedstock with the catalyst is too short, the yields of ethylene and propylene are disadvantageously lowered.
Due to the high reactivity of olefins, deposition of carbonaceous material (coke) on the surface of a zeolite-containing catalyst (hereinafter, this deposition of coke is referred to as coking) is likely to occur during the catalytic conversion reaction of a hydrocarbon feedstock comprised of olefins. Therefore, when the conversion reaction is continuously performed, the catalyst is deactivated due to coking (hereinafter, this type of the deactivation of the catalyst is frequently referred to as "coking deactivation"), so that the catalytic activity is easily lowered.
In general, when a catalyst is deactivated by coking, the activity of the catalyst can be restored by heating the deactivated catalyst in the presence of oxygen-containing gas so that the coke accumulated on the catalyst is burnt off. However, when the regeneration operation is repeatedly conducted, the catalytic activity can no longer be satisfactorily restored. The reason why the catalytic activity can no longer be satisfactorily restored is as follows. In the above-mentioned regeneration operation, in which the coke accumulated on the zeolite catalyst is burnt off, steam is generated. When a zeolite is heated in the presence of the generated steam, the aluminum atoms in the zeolite, which are active sites of the zeolite, are eliminated from the crystal framework of the zeolite by hydrolysis reaction, so that the zeolite-containing catalyst is permanently deactivated (hereinafter, this type of the deactivation of the catalyst is frequently referred to as regeneration deactivations).
As mentioned above, especially when a catalytic conversion reaction of a hydrocarbon feedstock comprised of olefins is performed using a zeolite-containing catalyst, the catalyst is likely to suffer coking. In that case, it becomes necessary to frequently perform the above-mentioned regeneration operation and, therefore, the regeneration deactivation of the catalyst is most likely to occur.
Unexamined Japanese Patent Application Laid-Open Specification No. 49-41322 (corresponding to British Patent No. 1381427) discloses a method for converting paraffins, olefins and/or cycloparaffins (naphthenes), each having 5 or more carbon atoms, to aromatic hydrocarbons, ethylene and propylene, in which a proton form ZSM-5 zeolite is used as a catalyst. However, in this method, the yields of ethylene and propylene are low, whereas aromatic hydrocarbons are obtained in relatively high yields.
Unexamined Japanese Patent Application Laid-Open Specification No. 50-49233 (corresponding to British Patent No. 1394979) discloses a method for converting C.sub.2 -C.sub.4 olefins and paraffins to aromatic hydrocarbons, ethylene and propylene, in which a proton form ZSM-5 zeolite is used as a catalyst. However, also in this method, the yields of ethylene and propylene are low, whereas aromatic hydrocarbons are produced in relatively high yields.
U.S. Pat. Nos. 4,527,001 and 4,613,721 disclose a method for converting butene to ethylene and propylene, in which an aluminophosphate molecular sieve is used. However, also in this method, the yields of ethylene and propylene are low.
Unexamined Japanese Patent Application Laid-Open Specification No. 3-27327 (corresponding to U.S. Pat. No. 5,043,522) discloses a method for producing ethylene and propylene, which comprises contacting a hydrocarbon feedstock comprised of a mixture of paraffins and olefins (each having 4 or more carbon atoms) with a proton form ZSM-5 zeolite as a catalyst, wherein the mixture has a specific composition. However, in this method, the conversion of the hydrocarbon feedstock is low, so that it is required to recycle a large amount of unreacted hydrocarbon feedstock.
Unexamined Japanese Patent Application Laid-Open Specification No. 6-73382 (corresponding to U.S. Pat. No. 5,171,921) discloses a method for converting C.sub.3 -C.sub.20 hydrocarbons to ethylene and propylene, in which a specific proton form ZSM-5 zeolite containing phosphorus is used as a catalyst. However, in this method, with respect to the conversion reaction results in the case where only olefins are used as a feedstock, the performance of the catalyst is evaluated only in the initial stage of the conversion reaction, namely, 1 minute after the start of the feeding of the feedstock.
As a feature common to all of the above-mentioned conventional methods, there can be mentioned the use of a proton form zeolite as a catalyst. In general, a proton form zeolite has strong acidity, and such a catalyst is likely to cause successive conversion reactions of ethylene and propylene to form aromatic hydrocarbons. Thus, in all of the above-mentioned methods, it is difficult to improve the yields of ethylene and propylene. Further, when a hydrocarbon feedstock comprised of olefins is used, a proton form zeolite is likely to suffer not only coking deactivation but also regeneration deactivation.
International Patent Application publication No. W096/13331 discloses a zeolite catalyst containing no proton, and a method for converting a hydrocarbon feedstock to ethylene, propylene and monocyclic aromatic hydrocarbons in presence of the zeolite catalyst containing no proton.
The catalyst used in this method is advantageous in that the regeneration deactivation of the catalyst is suppressed. However, the problem of occurrence of the coking deactivation of the catalyst remains unsolved. Therefore, when a hydrocarbon feedstock containing a large amount of olefins is used as a feedstock, the catalyst is likely to suffer coking deactivation.
Further, in this method, conversion reactions of paraffins occur simultaneously with the conversion reactions of olefins. For this reason, large amounts of various types of gases by-produced during the conversion reactions of paraffins (i.e., so-called "off gases"), such as hydrogen and methane, get mixed into the reaction mixture. Therefore, this method is disadvantageous in that the separation of ethylene and propylene from the reaction mixture is difficult.
In addition, the above-mentioned conversion reactions of paraffins are endothermic, so that it is required to supply the reactor with a large amount of heat. Therefore, in this method, a complicated, expensive reaction system is required.